In order to change the properties of intrinsic semiconductors a small
amount of some other material is added to it. The process of adding
other material to the crystal of intrinsic semiconductors to improve its
conductivity is called doping. The impurity added is called dopant.
Doped semiconductor material is called semiconductor. The doping
increases the conductivity of the basic intrinsic semiconductors hence
the extrinsic semiconductors are used in practice for manufacturing of
various electronic devices such as diodes, transistors etc.

Depending upon the type of impurities, the two types of extrinsic semiconductors are,

1. n-type and 2. p-type

1.1 Types of Impurities

The impurity material having five valence electrons is called
pentavalent atom. When this is added to an intrinsic semiconductor, it
is called donor doping as each impurity atom donates one free electron
to an intrinsic material. Such an impurity is called donor impurity. The
examples of such impurity are arsenic, bismuth, phosphorous etc. This
create an extrinsic semiconductor with large number of free electrons,
called n-type semiconductor.
Another type of impurity used
is trivalent atom which has only three valence electronics. Such an
impurity. When this is added to an intrinsic semiconductor, it creats
more holes and ready to accept an electron hence the doping is called
acceptor doping. The examples of such impurity are gallium, indium and
boron. The resulting extrinsic semiconductor with large number of holes
is called p-type semiconductor.